For a 125 I would put at least 2 more 2x4's in the middle just to be on the safe side. Plus that 2x4 in the back will give more attachment points for the plywood. You are going to have doors on there anyway (I'm assuming) so no reason not to add some center bracing.
I know we are talking compressive forces and yes, I think 4 2x4's would be able to handle it, but with the cost is it really worth risking?
IMO no...